The present invention relates to a method for controlling a printing press.
In a printing press, a plurality of cylinders are set into rotation during the printing operation. Since the printing of a printing substrate takes place in a continuous process during which the printing substrate sequentially passes the individual cylinders, the rotary motions of the cylinders have to be synchronized. This is generally also true for cylinders which do not come directly in contact with the printing substrate, such as the ink transfer cylinders. The synchronization of the individual cylinders is often accomplished by providing a single drive motor which, via mechanical coupling means, such as shafts, gear wheels, chains and couplings, drives all cylinders to be driven.
The mechanical coupling of the individual cylinders results in a vibratory system having a very complex structure so that a complicated pattern of vibratory motions is superimposed on the rotary motions of the cylinders. The vibratory motions can have a negative influence on the print quality. The negative influence has a particularly strong effect if the printing press contains at least one cylinder which is, completely or partially, mechanically and thus vibrationally decoupled from the remaining cylinders of the printing press and is provided with a separate drive so that the printing press is composed of a first and a second subsystem. The second subsystem performs the already mentioned vibration pattern whereas the first subsystem possibly does not vibrate or only slightly vibrates, or does not vibrate in sync with the second subsystem. This can result in a relatively abrupt transition in the vibratory motion between adjacent cylinders of the two subsystems.
There are already known different measures which are intended to reduce the negative effects of the machine vibrations or to ensure a good synchronization of the components of the printing press. For example, German Patent Application No. 197 40 153 A1 describes a printing press which has a plurality of electric motors, each motor driving a subsystem of the printing press. The actual angular velocity of each electric motor is controllable by a separate control circuit, respectively. The control circuit contains an observer which obtains an observed nominal load torque and an observed nominal angular velocity for the control from the actual angular velocity or the actual angle of rotation and the nominal torque of the respective electric motor.
German Patent Application No. 42 28 506 A1 discloses a method and a drive for a printing press having a plurality of printing units, the printing units being coupled to each other via a gear train and each printing press unit being associated with a drive motor which supplies power to the gear train. The first drive motor supplies a surplus of power to the gear train, ensuring a constant direction of the power flow in the gear train. The last drive motor compensates for the surplus of power.
German Patent Application No. 44 12 945 A1 describes a device for damping mechanical vibrations of printing machines, in which at least one vibration pick-up controls at least one actuating member in such a manner that the vibrations detected by the vibration pick-up are damped.
German Patent Application No. 197 42 461 A1 discloses a device and a method for driving printing presses using a plurality of motors which are arranged in a decoupled manner. In order to synchronize a plurality of printing unit groups that are each provided with a separate drive motor, in each case at least one transfer station having a separately controllable drive is provided between the printing unit groups. Through the transfer station, phase synchronism is first established with respect to the printing unit group arranged upstream of the transfer station and then with respect to the printing unit group arranged downstream thereof.
German Patent Application No. 198 26 338 A1 discloses a drive system for a printing press having a plurality of printing units which are provided with separate drives. In order to synchronize the gripper bridges during sheet transfer between the printing units, provision is made for a regulating device by which the phase relation of the machine-related fluctuations in the circumferential speed of the gripper bridges is measured and shifted in such a manner that a maximum correspondence in terms of location and time is accomplished during sheet transfer. Generally, the synchronization of the adjacent gripper bridges is limited to the moment of sheet transfer because optimization is carried out specifically for this moment.
German Patent Application No. 199 14 627 A1 describes a method and a device for compensating for rotational vibrations of a printing press. The excitation of vibrations is compensated for through the superposition of counter-torques. The counter-torques are superimposed at a location of the drive train of the printing press where one of the natural modes of the printing press is unequal to zero. In this context, the counter-torques are superimposed in such a manner that the vibration is maximally reduced.
Depending on the specific conditions, the known methods and devices already yield good results. However, the strongly varying excitation torques, which are frequently to be observed in printing presses and which place very high demands on the dynamics of the measures provided, generally create problems.